Email spam, also known as junk email or unsolicited bulk email (UBE), is a subset of spam that involves nearly identical messages sent to numerous recipients by email. Definitions of spam usually include the aspects that email is unsolicited and sent in bulk. One subset of UBE is UCE (unsolicited commercial email). The opposite of "spam", email which one wants, is called "ham", usually when referring to a message's automated analysis (such as Bayesian filtering).

Email spam has steadily grown since the early 1990s. Botnets, networks of virus-infected computers, are used to send about 80% of spam. Since the expense of the spam is borne mostly by the recipient, it is effectively postage due advertising.

The legal status of spam varies from one jurisdiction to another. In the United States, spam was declared to be legal by the CAN-SPAM Act of 2003 provided the message adheres to certain specifications. ISPs have attempted to recover the cost of spam through lawsuits against spammers, although they have been mostly unsuccessful in collecting damages despite winning in court.

Spammers collect email addresses from chat rooms, websites, customer lists, newsgroups, and viruses which harvest users' address books, and are sold to other spammers. They also use a practice known as "email appending" or "epending" in which they use known information about their target (such as a postal address) to search for the target's email address. Much of spam is sent to invalid email addresses. Spam averages 78% of all email sent. According to the Message Anti-Abuse Working Group, the amount of spam email was between 88–92% of email messages sent in the first half of 2010.

To prevent e-mail spam (aka unsolicited bulk email), both end users and administrators of e-mail systems use various anti-spam techniques. Some of these techniques have been embedded in products, services and software to ease the burden on users and administrators. No one technique is a complete solution to the spam problem, and each has trade-offs between incorrectly rejecting legitimate e-mail vs. not rejecting all spam, and the associated costs in time and effort.

Anti-spam techniques can be broken into four broad categories: those that require actions by individuals, those that can be automated by e-mail administrators, those that can be automated by e-mail senders and those employed by researchers and law enforcement officials.

Reducing or Preventing E-Mail Spam

There are a number of techniques that individuals can use to restrict the availability of their e-mail addresses, reducing or preventing their attractiveness to spam.

Discretion

Sharing an email address only among a limited group of correspondents is one way to limit spam. This method relies on the discretion of all members of the group, as disclosing email addresses outside the group circumvents the trust relationship of the group. For this reason, forwarding messages to recipients who don't know one another should be avoided. When it is absolutely necessary to forward messages to recipients who don't know one another, it is good practice to list the recipient names all after "bcc:" instead of after "to:” This practice avoids the scenario where unscrupulous recipients might compile a list of email addresses for spamming purposes. This practice also reduces the risk of the address being distributed by computers affected with email address harvesting malware. However, once the privacy of the email address is lost by divulgence, it cannot likely be regained.

Address munging

Posting anonymously, or with a fake name and address, is one way to avoid e-mail address harvesting, but users should ensure that the fake address is not valid. Users who want to receive legitimate email regarding their posts or Web sites can alter their addresses so humans can figure out but spammers cannot. For instance, joe@example.com might post as joeNOS@PAM.invalid.example.com. Address munging, however, can cause legitimate replies to be lost. If it's not the user's valid address, it has to be truly invalid, otherwise someone or some server will still get the spam for it. Other ways use transparent address munging to avoid this by allowing users to see the actual address but obfuscate it from automated email harvesters with methods such as displaying all or part of the e-mail address on a web page as an image, a text logo shrunken to normal size using in-line CSS, or as jumbled text with the order of characters restored using CSS.

Avoid responding to spam

Spammers often regard responses to their messages—even responses like "Don't spam me"—as confirmation that an email address is valid. Likewise, many spam messages contain Web links or addresses which the user is directed to follow to be removed from the spammer's mailing list. In several cases, spam-fighters have tested these links, confirming they do not lead to the recipient address's removal—if anything, they lead to more spam. This removal request of filing a complaint may get the address list washed. To lower complaints so the spammer can stay active before having to acquire new accounts and/or internet provider.

Sender addresses are often forged in spam messages, including using the recipient's own address as the forged sender address, so that responding to spam may result in failed deliveries or may reach innocent e-mail users whose addresses have been abused.

In Usenet, it is widely considered even more important to avoid responding to spam. Many ISPs have software that seek and destroy duplicate messages. Someone may see a spam and respond to it before it is cancelled by their server, which can have the effect of reposting the spam for them; since it is not a duplicate, the reposted copy will last longer. Replying may also cause the poster to be falsely linked to as part of the spam message.

Contact forms

Contact forms allow users to send email by filling out forms in a web browser. The web server takes the form data, forwarding it to an email address. Users never see the email address. Such forms, however, are sometimes inconvenient to users, as they are not able to use their preferred e-mail client, risk entering a faulty reply address, and are typically not notified about delivery problems. Further, contact forms have the drawback that they require a website that supports server side scripts. Finally, if the software used to run the contact forms is badly designed, it can become a spam tool in its own right. Additionally, some spammers have begun to send spam using the contact form.

Disable HTML in e-mail

Many modern mail programs incorporate Web browser functionality, such as the display of HTML, URLs, and images. This can easily expose the user to offensive images in spam. In addition, spam written in HTML can contain web bugs which allow spammers to see that the e-mail address is valid and that the message has not been caught in spam filters. JavaScript programs can be used to direct the user's Web browser to an advertised page, or to make the spam message difficult to close or delete. Spam messages have contained attacks upon security vulnerabilities in the HTML renderer, using these holes to install spyware. (Some computer viruses are borne by the same mechanisms.)

Mail clients which do not automatically download and display HTML, images or attachments, have fewer risks, as do clients who have been configured to not display these by default.

Disposable e-mail addresses

An email user may sometimes need to give an address to a site without complete assurance that the site owner will not use it for sending spam. One way to mitigate the risk is to provide a disposable email address—a temporary address which the user can disable or abandon which forwards email to a real account. A number of services provide disposable address forwarding. Addresses can be manually disabled, can expire after a given time interval, or can expire after a certain number of messages have been forwarded. Site owners that fail to keep addresses they have gathered confidential have found themselves in legal jeopardy due to the ability of disposable email address users to trace which website passed on their email without permission.

Ham passwords

Systems that use ham passwords ask unrecognized senders to include in their email a password that demonstrates that the email message is a "ham" (not spam) message. Typically the email address and ham password would be described on a web page, and the ham password would be included in the "subject" line of an email address. Ham passwords are often combined with filtering systems, to counter the risk that a filtering system will accidentally identify a ham message as a spam message.

The "plus addressing" technique appends a password to the "username" part of the email address.

Reporting spam

Tracking down a spammer's ISP and reporting the offense can lead to the spammer's service being terminated. Unfortunately, it can be difficult to track down the spammer—and while there are some online tools to assist, they are not always accurate. Occasionally, spammers employ their own netblocks. In this case, the abuse contact for the netblock can be the spammer itself and can confirm your address.

Examples of these online tools are SpamCop and Network Abuse Clearinghouse. They provide automated or semi-automated means to report spam to ISPs. Some spam-fighters regard them as inaccurate compared to what an expert in the email system can do; however, most email users are not experts.

A free tool called Complainterator may be used in the reporting of spam. The Complainterator will send an automatically generated complaint to the registrar of the spamming domain and the registrar of its name servers.

Historically, reporting spam in this way has not seriously abated spam, since the spammers simply move their operation to another URL, ISP or network of IP addresses.

Consumers may also forward "unwanted or deceptive spam" to an email address (spam@uce.gov) maintained by the FTC. The database collected is used to prosecute perpetrators of scam or deceptive advertising.

An alternative to contacting ISPs is to contact the registrar of a domain name that has used in spam e-mail. Registrars, as ICANN-accredited administrative organizations, are obliged to uphold certain rules and regulations, and have the resources necessary for dealing with abuse complaints.